1. Field of the Invention
The invention relates to an exhaust gas recirculation system for an internal combustion engine, and a method for controlling the same.
2. Description of the Related Art
An exhaust gas recirculation (EGR) system that recirculates a portion of the exhaust gas back to an intake system of an internal combustion engine has been suggested in order to reduce the amount of nitrogen oxide (NOx) generated in the process of burning fuel in the internal combustion engine.
A technology that makes it possible to perform EGR in a broader operating range of an internal combustion engine is described in, for example, Japanese Patent Application Publication No. 2004-150319 (JP-A-2004-150319). According to this technology, a high-pressure EGR unit and a low-pressure EGR unit are provided, and EGR is performed while the EGR unit used for EGR is changed between the high-pressure EGR unit and the low-pressure EGR unit or both the high-pressure EGR unit and the low-pressure EGR unit are used in combination, based on the operation mode of the internal combustion engine. The high-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between an exhaust passage, at a portion upstream of a turbine of a turbocharger, and an intake passage, at a portion downstream of a compressor of the turbocharger. The low-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor. Other related technologies are described in, for example, Japanese Patent Application Publication No. 2002-21625 (JP-A-2002-21625) and Japanese Patent Application Publication No. 2005-76456 (JP-A-2005-76456).
In the EGR system that performs EGR using the high-pressure EGR unit and the low-pressure EGR unit in combination, the amount of EGR gas recirculated back to the internal combustion engine is controlled by adjusting the opening amount of a high-pressure EGR valve, which regulates the flow rate of high-pressure EGR gas flowing through the high-pressure EGR passage, and the opening amount of the low-pressure EGR valve, which regulates the flow rate of low-pressure EGR gas flowing through the low-pressure EGR passage.
Usually, a specified value of the high-pressure EGR valve and a specified value of the low-pressure EGR valve are set in such a manner that the amount of EGR gas recirculated back to the internal combustion engine is decreased as the fuel injection amount is increased in accordance with a change in the operation mode of the internal combustion engine to an operation mode in which a higher load is applied to the internal combustion engine. However, if the opening amount of the high-pressure EGR valve and the opening amount of the low-pressure EGR valve are adjusted to the specified values (i.e., the open-loop control is executed) in the engine speed-up transitional operation period in which the operation mode of the internal combustion engine is changing to the operation mode in which a higher load is applied to the internal combustion engine, the amount of EGR gas that is recirculated back to the internal combustion engine may be excessive. This may occur mainly because the low-pressure EGR gas amount does not change promptly in response to the control over the low-pressure EGR valve opening amount. As a result, a balance between the amount of air taken in the internal combustion engine and the fuel injection amount is not appropriately maintained, resulting in an increase in the amount of smoke.
The above-described inconvenience occurs due to the following reason. The amount of low-pressure EGR gas that is present in the flow path from the low-pressure EGR valve to cylinders of the internal combustion engine is relatively large, because the low-pressure EGR passage is long. Accordingly, even when the opening amount of the low-pressure EGR valve is changed, the amount of low-pressure EGR gas that is actually taken in the cylinders does not match the low-pressure EGR gas amount corresponding to the changed opening amount of the low-pressure EGR valve until the above-described residual low-pressure EGR gas is entirely taken in the cylinders.
In the engine speed-up transitional operation period in which the operation mode of the internal combustion engine is changing, the high-pressure EGR valve opening amount and the low-pressure EGR valve opening amount may be controlled in a feedback manner. However, in this case as well, the high-pressure EGR gas amount is appropriately controlled because it changes promptly in response to a change in the opening amount of the high-pressure EGR valve, whereas the low-pressure EGR gas amount does not change promptly in response to a change in the opening amount of the low-pressure EGR valve. Therefore, hunting occurs in the control over the amount of EGR gas recirculated back to the internal combustion engine that is, in the amount of air taken in the internal combustion engine, resulting in an increase in the amount of smoke and deterioration in the drivability. Such an inconvenience occurs because the speed of response to the control greatly differs between the two parameters which are controlled in a feedback manner at the same time.